FIG. 11 shows how particles are accelerated in a cyclotron. A cyclotron is typically formed of an electromagnet and dee electrodes 1. The whole structure of dee electrode 1 is accommodated in an evacuated box. Accelerated particles including protons are generated in an ion source located at cyclotron core 3. The ion source located in an evacuated box is often called an internal ion source. On the other hand, there is another type of cyclotron, where the ion source is disposed outside the cyclotron and beams are conveyed from the ion source to cyclotron core 3. The structure has an advantage of being accessible to the ion source in maintenance work, with the vacuum condition of the evacuated box maintained. Such an ion source disposed outside the evacuated box is called an external ion source. In a cyclotron having an external ion source, beams conveyed from the external ion source are fed into cyclotron core 3 and then accelerated.
In FIG. 11, RF acceleration voltage is applied to dee electrodes 1. Each time passing across gap 2 disposed between dee electrodes 1, a particle gains energy corresponding to the electric field between dee electrodes 1. Because the electric field does not penetrate deep into dee electrodes 1, the particle traveling through the electrodes has no influence of the electric field. When reaching gap 2 after semicircle traveling, the particle receives a 180° phase-shifted RF acceleration voltage, so that the particle gains energy from the electric field. In this way, starting from cyclotron core 3, the particle gains energy from the electric field each time it reaches gap 2 after a semicircle travel, and accordingly, the orbital radius of the traveling particle is getting larger. At a position close to the circumference of the magnetic pole, deflector 4, which is a high voltage electrode for capturing beams, is disposed. The particle entered into deflector 4 is retrieved, by radially-outward force, from the magnetic field of the cyclotron. Generally, a particle is supposed to be accelerated 1000 times during the 500 times go-around. Particles having difference in phase with respect to the RF acceleration voltage at the start from cyclotron core 3 are to be given different acceleration voltage, which invites variations in energy to be gained by particles and variations in orbits of the particles. The variations in orbits lower the efficiency of retrieving beams, and the variations in energy degrade the quality of retrieved beam 5. To avoid the inconveniencies above, an improved cyclotron capable of keeping the phase width of a beam small at the first-turn of the acceleration process has been needed for providing beams with high quality.
Responding to the demand, various methods of minimizing the variations in phase widths of beams have been introduced. For example, a cyclotron having a phase slit is disclosed in one suggestion (see Recent Developments at the Osaka RCNP 230-cm Cyclotron and a Proposal for a New Ring Accelerator, IEEE Trans NS-26, 2, pp. 1904-1911). According to the method, after leaving the internal ion source and passing across gap 2 twice, the particles undergo screening by the phase slit-undesired particles are blocked and not allowed to pass through. The phase slit has a beam blocking section movable disposed in a radial direction from cyclotron core 3 with respect to the orbit of the particle centered in a beam.
Explanations hereinafter will be given with reference to FIGS. 12 and 13. FIG. 12 illustrates RF acceleration voltage to be applied at gap 2 to a beam having a time lag equivalent to ±40° of the phase of the voltage. The description will be given on acceleration of a particle bearing positive charge. The application of voltage is usually controlled so that RF acceleration voltage with a phase of 270° is applied to the particle traveling in the middle of the time lag when the mid particle passes across gap 2. That is, the mid particle gains energy at point A3 in FIG. 12. Particles traveling 40° behind, and 20° behind in phase with respect to the mid particle gain energy for acceleration at point A1 and A2, respectively. On the other hand, particles traveling 40° ahead, and 20° ahead in phase gain energy at point A5 and A4, respectively. The orbit taken by a particle depends on the amount of energy gained by the particle. The orbit of a particle at the first turn is easily explained.
FIG. 13 illustrates the orbit of an accelerated particle at the first turn in a cyclotron. Each particle gains energy with the application of acceleration voltage at point An (in FIG. 12), and takes the orbit n (where, n takes 1 to 5). The mid particle gains energy at the highest acceleration voltage, and therefore the particle takes the orbit having the largest orbital radius; the mid particle takes the outermost orbit 3. On the other hand, a phase-shifted particle gains energy smaller than the mid particle; accordingly, the orbital radius of the particle is smaller than that of the mid particle. Each particle accelerated at A2 and A4 takes the same orbit, i.e., orbit 2 (4), and similarly, each particle accelerated at A1 and A5 takes the same orbit, i.e., orbit 1 (5). Many of conventional phase control structure, such as phase slit 6 in FIG. 13, have used the difference in orbits described above. That is, a conventional cyclotron often contains a beam blocking section disposed movable in the radially-outward direction from cyclotron core 3.
FIG. 13 shows oval-shaped blocking section 7 as an example. Rotating oval blocking section 7 can change beam control. For example, when blocking section 7 is disposed at the position as shown in FIG. 13, the particles traveling along orbit 1 (5) are blocked, whereas the particles taking orbit 3 and orbit 2 (4) continue to travel, so that the phase widths of the beam are limited within ±20°. In addition to blocking section 7, disposing conventional phase slit 14 can also block undesired particles taking orbits in a direction of radially-outside from the mid orbit. When a cyclotron employs an internal ion source, phase widths of beams can be limited within a desired range, whereby beams with consistent energy can be obtained.
However, the aforementioned phase slit produces an inconvenience in a cyclotron employing an external ion source; disposing conventional phase slits, such as phase slits 6 and 14, lowers beam permeability to approx. 1/50, and therefore weakens beam current. The problem probably comes from the difference in incidence energy of particles to be fed into the cyclotron. In a cyclotron having an internal ion source, particles are drawn out from the ion source by RF acceleration voltage applied to gap 2—the incident energy of a particle is nearly zero. In contrast, in a cyclotron having an external ion source, particles are drawn out from the ion source by voltage applied to an interconnect electrode of the ion source—a particle already has an energy before being fed into the cyclotron. Generally, having 10 keV or more energy, protons are fed into a cyclotron via an axial incidence system. Due to the incident energy, the difference in energy among the particles relative to an absolute value of energy becomes small. Accordingly, the difference in orbits taken by the particles becomes narrow. Therefore, the conventional beam control method—where the control of phase widths is relied on the difference in orbits caused by the difference in energy gained by a particle at the gap—is not effective in blocking out undesired particles.
To address the problem above, suggestions on a phase slit in a cyclotron employing an external ion source are introduced, for example, in Recent Developments of Ring Cyclotron, Nucleus Research Vol. 36, No. 2, pp. 3-15, 1991, and in The Research Center for Nuclear Physics Ring Cyclotron, Proceedings of the 1993 Particle Accelerator Conference Volume 3 of 5, pp. 1650-1654.
FIG. 14 shows conventional phase slit 8 introduced in a suggestion above. Conventional phase slit 8 has electrode 9 and electrode 10. Electrode 9 has an opening in a direction of a core of a cyclotron, and electrode 10 is disposed in a radially outside position of the cyclotron so as to face electrode 9.
While the particles are traveling through dee electrode 1 after first passing of gap 2 since the start at cyclotron core 3, the particles reach dee electrode 1, and undesired particles of them are blocked by electrodes 9 and 10. Usually, the particles have no effect from electric field. However, through the opening of electrode 9, electric field leaks into dee electrode 1, so that the particles gain energy from the leakage electric field that is on its way changing from minus to plus of RF acceleration voltage. The leakage electric field affects on the beam with a time lag so as to replace distribution of time with distribution of orbital radius. As a result, at the exit of the phase slit, the beam has a stretch in a radial direction of the cyclotron.
FIG. 14 shows the orbits ±15°-shifted in phase from the orbit of the mid particle. The phase-shifted particles are blocked by electrode 9 with an opening and electrode 10 disposed in a radially-outward position of the cyclotron so as to be opposite to electrode 9. The structure of FIG. 14 can block out particles phase-shifted ±15° or more. The particles with a phase shift of ±15° were assumed to take inward and outward orbits, being equally away from the orbit of the mid particle. Considering this, the two electrodes were properly shaped and fixed so as to contact with the orbits having a phase shift of ±15° from the orbit of the mid particle. An experiment was done by using conventional phase slit 8 and the result is disclosed in Operation of RCNPAVF Cyclotron, RGNP Annual Report 1991, pp. 207-210. According to the report, the beam permeability when a cyclotron employs an external ion source is improved to ⅕- 1/7.
Generally, a phase-width control that can provide a larger beam current for a consistent phase width is more preferable. Therefore, the phase width control method capable of providing a consistent phase width and increased beam current has been demanded. An effort to address the problem is introduced in A NEW BEAM PHASE SELECTOR FOR THE AVF CYCLOTRON, RCNP Annual Report 1996, pp. 178-181. In the report, the orbit of a particle is calculated by a calculator through three-dimension field analysis of the core area of a cyclotron. According to the result of the orbit calculation, beam permeability measured 1/16- 1/30, having no direct contribution to improvement in efficiency of performance.
The needs for an improved method and device of selecting phase width—not only obtaining a consistent phase width but also providing improved beam permeability for larger beam current—have been raised.